IT IS smaller, patchier and thinner than ever – and rotten in parts. The extent of the Arctic ice cap has hit a record low, and the consequences of what is arguably the greatest environmental change in human history will extend far beyond the North Pole.

For at least 3 million years, and most likely 13 million, says Louis Fortier of the University of Laval in Quebec City, Canada, the Arctic Ocean has been covered by a thick, floating ice cap, the breadth of which fluctuates with the seasons …